The interactions between stromal components and epithelial cells are known to be important in determining breast cancer risk and progression. However, many of the molecular mechanisms that underpin these interactions are unknown. We have recently identified an integrated multi-cellular program that controls manifold stromal alterations in breast cancer. Modulation of this program induces the concomitant appearance of many phenotypic alterations often observed in the stromal components surrounding a tumor: low adipocyte content, increased deposition of extra-cellular matrix (ECM) proteins and increased numbers of fibroblasts and vascular cells. These visual changes, referred to as desmoplasia, are induced by the secretion of cytokines that are instrumental in tumor progression. We have established a direct causal relationship between acquisition of these desmoplastic features and repression of the receptor CD36 in vitro and in vivo. Strikingly, CD36 repression is observed in all stromal cell types normally expressing CD36, fibroblasts, endothelial cells, macrophages and adipocytes. This repression occurs throughout breast tumor stroma compared to its adjacent histologically normal tissue. Widespread repression of CD36 has many functional implications since CD36 coordinately modulates adipocyte differentiation, angiogenesis, matrix deposition, cell-ECM interactions, and immune signaling, all of which constitute pro-tumorigenic phenotypes. Consistent with this, CD36 tumor levels are inversely correlated with tumor size and grade. Importantly, these stromal changes are seen in breast tissues in the absence of any malignancy, supporting that it is an early event in the tumorigenic process. We have gathered compelling evidence for the cytokine activin A repressing CD36, thus providing a mechanistic link between engagement of the DNA damage-Activin A pathway identified by us in damaged pre-malignant epithelial cells (vHMECs) and activation of the multicellular pro-tumorigenic stromal program characterized by CD36 repression. In this proposal, we wish to demonstrate the extent of the extrinsic impact of DNA damage within the epithelium on the stromal network described above. In Specific Aim 1, we will investigate if damaged vHMECs, and malignant cells that typify different breast tumor subtypes, can repress CD36 through activin A in all stromal cell types that express CD36 and if tumorigenic phenotypes are induced in each cell type as a result of CD36 repression in vitro. In Specific Aim 2, we will evaluate whether a telomere malfunction (DNA damage) murine model exhibits cd36-dependent desmoplastic and stromal tumorigenic phenotypes in vivo. In Specific Aim 3, we will evaluate the contribution of each stromal cell type to these CD36-dependent tumorigenic phenotypes in vivo using a mouse xenograft model optimized in our laboratory. Finally, in Specific Aim 4, we will search for novel factors secreted by damaged vHMECs that can, like activin A, repress CD36 and promote CD36-dependent stromal tumorigenic phenotypes. Answers to these questions will provide clinicians with novel tools for prevention or reversion of the malignant state in human breast tissue.